The invention relates to an actuator arrangement of the type comprising a piezoelectric element. In particular, but not exclusively, the invention relates to an actuator arrangement for use in controlling valve needle movement in a fuel injector.
Piezoelectric actuators are used in fuel injectors to control movement of a valve needle within a fuel injector nozzle body, movement of the valve needle being used to control the delivery of fuel into an engine cylinder or other combustion space.
FIG. 1 is a schematic diagram of a conventional piezoelectric actuator arrangement of this type. The arrangement includes a piezoelectric stack 10 formed from a plurality of piezoelectric elements. One end of the piezoelectric stack 10 abuts a part of a fuel injector housing 11, the other end of the piezoelectric stack 10 being in engagement with an input piston 12, having a diameter di, which is moveable within a bore 13 provided in the housing 11. The axial length of the piezoelectric stack 10 is controlled by controlling the energization level of a piezoelectric stack 10, the energization level of the piezoelectric stack 10 being controlled by applying a voltage across the stack 10. The actuator arrangement also includes an output piston 16 having a diameter do which is less than the diameter di of the input piston. The output piston 16 is moveable with the fuel injector valve needle (not shown), the output piston 16 being exposed to fuel pressure within a control chamber 14 which is supplied with fuel, in use, either through leakage or by means of an appropriate valve arrangement (not shown). The piezoelectric stack 10 may be housed directly within the fuel injector housing 11 or may be housed within a chamber in communication with a low pressure fuel reservoir or drain. The input piston 12 and the bore 13 form a substantially fluid tight seal such that, in use, little or no fuel is able to escape from the control chamber 14.
In use, assuming a substantially constant load is applied to the end face of the output piston 16 remote from the control chamber 14, when the length of the piezoelectric stack 10 is increased, the input piston 12 moves inwardly within the bore 13. As fuel within the control chamber 14 is substantially incompressible, the volume of fuel within the control chamber 14 remains substantially constant during inward movement of the input piston 12. As a result, the output piston 16 will be displaced by an amount which is greater than the displacement of the input piston 12 by the ratio di2/do2.
A problem exists in conventional actuator arrangements of the aforementioned type in that, under conditions in which a relatively high actuation force is required, fuel pressure within the control chamber 14 applies a relatively large load to the piezoelectric stack through the input piston 12. The relatively large load applied to the stack results in a reduced displacement of the piezoelectric stack 10. It is thought that this displacement suppression is due to domain switching of the piezoelectric dipoles which occurs as a direct result of uniaxial straining of the piezoelectric material lattice. In practice, piezoelectric materials which are relatively xe2x80x9csoftxe2x80x9d are often preferred to those which are relatively xe2x80x9chardxe2x80x9d as it is possible to achieve a greater displacement with such materials. However, the displacement of piezoelectrically soft materials is compromised to a greater extent due to domain switching than for relatively hard piezoelectric materials. Typically, relatively soft piezoelectric materials exhibit a pronounced displacement suppression at uniaxial compressive stresses in excess of 40 MPa which, for many applications, does not permit a sufficient actuation force, or actuation stroke, to be obtained.
It is an object of the present invention to provide an actuator arrangement which alleviates the aforementioned disadvantage.
According to a first aspect of the present invention, an actuator arrangement comprises a piezoelectric element formed from a piezoelectric material, a first end of the piezoelectric element cooperating with an input piston member slidable within a bore, a surface associated with the input piston member defining, in part, a control chamber for fluid, fluid pressure within the control chamber acting on a surface associated with an output piston member, the fluid within the chamber being substantially incompressible such that, in use, movement of the input piston member is transmitted to the output piston member, the piezoelectric element being arranged within a second chamber for fluid such that fluid pressure within the second chamber exerts a force on at least a part of the surface of the piezoelectric element which serves to oppose the load exerted on the piezoelectric element due to fluid pressure within the control chamber, thereby suppressing distortions in the piezoelectric material.
The actuator may include a single piezoelectric element or may include a stack of piezoelectric elements.
The invention provides the advantage that lattice distortions in the piezoelectric material, which would otherwise compromise displacement of the piezoelectric element, are suppressed. In particular, such distortions occur when relatively large loads are applied to the piezoelectric element or stack of piezoelectric elements. In the present invention, even when the load applied to the piezoelectric element is relatively high, displacement of the piezoelectric element is not compromised as such lattice distortions are suppressed.
The input piston member may have a diameter which is greater than that of the output piston member. In this case, the output piston member is caused to be displaced by a greater amount than the input piston member.
Alternatively, the input piston member may have a diameter which is smaller than that of the output piston member such that the output piston member is caused to be displaced by a smaller amount than the input piston member. This is particularly advantageous if the actuator is to be employed in applications where only relatively small displacements are required.
The actuator arrangement may include a clearance passage which permits fluid to escape from the control chamber to the second chamber, in use, such that fluid pressures within the second chamber and the control chamber tend to equalise. The clearance passage preferably has a restricted diameter to ensure fluid flow from the control chamber to the second chamber occurs at a relatively low rate. In this way, any loss of the actuation stroke due to the reduction in fluid pressure within the control chamber as fluid flows through the clearance passage is minimised.
The clearance passage may be defined by one or more drillings provided in a housing for the actuator arrangement.
Alternatively, the clearance passage may be defined between the input piston member and the bore within which the input piston member is slidable.
Alternatively, the control chamber may be substantially sealed from the second chamber, the actuator arrangement further including means for measuring fluid pressure within the control chamber and means for supplying fluid to the second chamber in response to the measured fluid pressure such that fluid pressure within the second chamber is maintained at a level substantially equal to that within the control chamber, in use.
This provides the advantage that, as fluid is unable to escape from the control chamber to the second chamber, there is no loss of actuation stroke. This is particularly important in applications for which a relatively long actuation stroke is required.
The actuator arrangement of the present invention may be used in a fuel injector arrangement to control movement of a fuel injector valve needle. The fluid within the control chamber may therefore conveniently take the form of fuel to be delivered by the injector arrangement.
According to a further aspect of the present invention, there is provided a fuel injector comprising an actuator arrangement as herein described, the fuel injector comprising a valve needle which is operable under the control of the actuator arrangement, the valve needle being engageable with a seating to control fuel delivery through an injector outlet, wherein the second chamber is arranged to receive pressurised fuel, in use, through an injector inlet.